The present invention relates to plasticating screws for use in extruders and injection molding machines and in particular to a mixing section on a plasticating screw that is capable of bidirectional distribution of inhomogeneities.
The extrusion and injection molding process, in common with most polymer processing operations, forms shapes from thermoplastic feedstock by softening or plasticating the material to a molten state, shaping the product in an extrusion die or in a closed mold, and solidifying the product by cooling. The properties of the molded product depend to a major extent upon the processing history of the material, and particularly upon the thermal and physical homogeneity of the molten feedstock prior to the forming step. Inhomogeneities in the feedstock can result in the loss of dimensional accuracy, lower physical properties, inconsistent product morphology and reduced product performance and service. Accordingly, processing of a homogeneous melt is a major concern in the injection molding and extrusion industry.
Conversion of the usually solid feedstock to the molten state is accomplished in the extrusion apparatus by the transfer of energy to the polymer from two sources, namely, from the heated barrel by the process of thermal conduction, and from the screw drive motor through the mechanism of viscous dissipation as the polymer is sheared during screw rotation. During shearing of the polymer, material is spatially mixed within the confines of the screw channel, and the extent of this mixing has a significant affect upon the final uniformity of the polymer melt. In the case of injection molding, since this is a cyclic process, the polymer plasticating process is strongly influenced by transient effects which result in differing proportions of the melting energy being supplied by conduction or dissipation mechanisms as the screw rotation operation progresses. This in turn produces a variation in the degree of mixing produced by the screw during rotation. In general, material delivered by the screw during the initial stages of rotation is well mixed whereas material delivered in the final stages of screw rotation is less well mixed. As a result, the uniformity of the melt tends to decrease during plastication. The problems of poorer mixing and the presence of inhomogeneities in the final product is also a problem in extrusion applications.
A number of solutions to the problem of poor mixing in extrusion and injection molding applications have been proposed. One solution is to increase the barrel length-to-diameter ratio of the extruder, which increases the residence time of the material within the extruder screw, thereby achieving better mixing and temperature uniformity. However, this solution is quite costly, and is therefore unacceptable in many situations. Another prior art solution to the problem of incomplete mixing is to provide a plurality of pegs or other protrusions in the screw channel near the discharge end to thereby promote mixing of the solid and molten plastic components.
Other prior art screw designs for extrusion and injection molding employ a plurality of helical channels that are separated by an undercut barrier flight, and the molten plastic is caused to flow back and forth over the barrier flight as the channel depth in the two channels cyclically increases and decreases in an out of phase relationship with each other. An example of an extrusion screw employing this design is disclosed in U.S. Pat. No. 4,273,417 issued to HPM Corporation of Mount Gilead, Ohio.
U.S. Pat. No. 4,405,239 discloses an extrusion screw having a pair of channels separated by flights, the latter being undercut at different locations along the length of the screw. This screw promotes mixing by providing mass transfer of the material in the back-direction, that is, in a direction toward the feed end of the screw. However, a mixing operation that only provides mass transfer in one direction can only provide a final distribution width one-half that of a mixing operation which provides mass transfer in both forward and reverse directions. Furthermore, a plasticating screw with back mixing only will cause the material in the screw channel to experience a higher average residence time, which can lead to additional undesirable degradation, crosslinking and the like.